May-June 2018 NPJ

Nuclear Plant Journal, May-June 2018 NuclearPlantJournal.com 43 (Continued on page 44) Innovation The repair method for performing both of the tasks had not been developed and a repair approach had to be designed for this unique remediation to keep the plant online. Innovation was the key to success for this project, and the process and tooling used to implement was First- of-a-Kind (FOAK). The unique repair technique involved drilling through the valve body; using “hot taps” to insert the stabilization pins, drilling through the valve disc and stem, and inserting a repair pin to connect the valve disc and stem together. This allowed the valve to be opened and CW pump C to be operational. The following innovative solution was implemented to keep the plant online:  The first step was to stabilize the valve to prevent inadvertent opening to ensure reliable 100% power production during peak summer season.  Position of the valve disc was unknown. This posed a problem in that stabilizing the disc with the pins had to interact with features of the disc to ensure it was truly held in place. The pin design, process and analysis had to account for this uncertainty.  The disc position was found to be different than anticipated. Since this was part of the design and analysis, the process was able to accommodate this variance.  An integrated hot tap and drilling fixture had to become part of the seal. This was a must in the design since the water could not be isolated and the valve was under a known pressure. A specialized oil derrick drill fixture was designed to allow drilling while under pressure, isolating the water keeping the system and process relatively dry.  The entire operation was done “blind” - there was no video, and operators had to use system gauges. The mockup allowed the operators an understanding of what to expect and cameras were added to the tool, however the clarity of the Mississippi River water precluded any visual evidence of the process. The use of the mockup allowed the operators a measure of confidence in trusting their gauges while working blind.  The use of a special drill bit for chip removal was incorporated from the mockup testing and proved to be efficient both technically and with schedule. The chips were found to be a problem and would at times bind the drill up halting the process. The special drill allowed for the chips to be removed and after mockup testing, showed that the process at site could be predictable.  All machining debris was removed via a specialty drill process leaving no foreign material.  A drilled 2” hole that was 5 feet thru 5 transitions and 3 different materials (cast, 304 and stainless steel). The drilling process had to be designed to go through the following parts of the valve assembly in the order: valve body, disc, stem, possible open hole where the original pin used to be (or remnants thereof), stem (again), and finally the opposite side of the disc. The design had to implement a spot face for the body, ability to breach the body of the valve, spot face the disc and finally bore through the valve.  The condition of the existing pins was unknown. As mentioned above, the pins could have been missing or part of the pin could have still been intact. The problem with the pin(s) missing was an interrupted cut due to the void in the material. The design had to take this into account. The process was proven on a spare 84" mockup which allowed the process to be predictable and increased the efficiency at site.  Tolerances were held to maintain engineering design values established in analytical models to maintain integrity. The design replaced two pins with one pin and the analysis, with some conservatism, allowed for this approach. Safety Over 200 hours were worked in a challenging environment performing a FOAK evolution. The work area was ~30ft. deep ~6-8 ft. wide and damp, lending to slip and trip hazards. In w w w . g r i f f o l y n . c o m • 1 . 8 0 0 . 2 31 . 6 0 7 4 IDEAL FOR CONSTRUCTION OUTAGE PROTECTION AND MAINTENANCE FME CUSTOM CONTAINMENT Stock rolls available in various sizes with same day shipping